1. Field
This invention relates to watermarking of digital images.
2. Background Art
Digital imaging allows images to be represented with digital data. With the growth in availability and access to the World Wide Web (WWW) and the widespread availability of digital imaging technology, the number of digital images and the frequency of their use has grown. Many applications provide accessibility to particular digital images or image collections to a large number of different users for purposes such as viewing, editing, or sharing. For instance, digital images of persons, animals, objects, and scenery, are often posted on the WWW by owners of those images with the intent of sharing the images with a select group of people or the public at large.
However, the accessibility of these digital images on the WWW makes it easier for others to copy and use them for unauthorized purposes. For example, a member of Google's Orkut social networking community may post an image to identify himself as the owner of material posted on his blog. Another person may copy this image and post it elsewhere purporting to be his own. Many examples exist, such as above, where digital images can be copied and used by persons other than the rightful owners.
One technique used to reduce unauthorized copying of images, is the technique of watermarking. The watermarking of a digital image includes the embedding of a digital watermark (referred to simply as a “watermark”) in that image. The watermark can be a digital signature, a user identifier, a logo, or similar value that can sufficiently identify the image as to its origin or ownership. It is highly desirable that the embedded watermark does not alter the appearance of the original image, i.e., the watermark should be substantially invisible to the human eye. The watermark perceivably alters the appearance of the original image in many of the conventional watermarking methods.
Digital images are generally posted on the WWW in a compressed form, such as, the Joint Photographic Experts Group (JPEG or JPG) compression format or other type of image compression format. Because image compression methods alter the values of selected image pixels, it is possible that the embedded watermark is affected in such a way as to alter the appearance of the image and/or to make the watermark unrecoverable. Therefore, it is also desired that a watermark be robust to survive high levels of compression, such as JPEG 60 or JPEG 80 compression.
Often, a person copying a digital image also engages in altering the image, for example, by cropping the original image. In order to be effective, an embedded watermark should also be detectable and/or retrievable from a copied image after such cropping has taken place.
Many approaches to digital watermarking have been proposed. For example, one approach involves the generation of a discrete courier transform (DCT) coefficient matrix (DCT matrix) for the image, dividing the coefficient matrix into one or more watermark blocks, and embedding a digital signature in each of the watermark blocks. Wen-Nung Lie, Guo-Shiang Lin, Chih-Liang Wu, and Ta-Chun Wang, “Robust Image Watermarking On the DCT Domain,” IEEE International Symposium on Circuits and Systems, Geneva, (2000), describes such an approach. The method described by Lie et al. embeds a watermark repetitively in different sections of an image in the corresponding DCT matrix. However, this method is vulnerable to cropping and is computationally expensive. For example, cropping a watermarked image having a watermark embedded using the Lin et. al. method, can prevent the cropped image from being recognized as a watermarked image. Also, the method described by Lie et. al., incurs a relatively high cost of computation by requiring identification of a substantial part of the originally embedded watermark in order to determine that the image was watermarked.
Therefore, improved methods and systems for watermarking of digital images are needed.